When speech/music signals are transmitted using a mobile communication system or a packet communication system represented by Internet communication, compression/encoding techniques are used to improve transmission efficiency of the speech/music signals. Recently, there are increasing needs for techniques capable of encoding multi-channel speech/music signals such as stereo signals as well as monaural signals even though speech/music signal is being encoded at a low bit rate.
For example, as a technique of encoding two-channel signals (stereo signals) including a left channel signal (hereinafter, referred to as an L signal) and a right channel signal (hereinafter, referred to as an R signal), there are known background arts such as a middle/side (M/S) stereo encoding scheme and an intensity stereo encoding scheme. Here, the M/S encoding scheme will be described shortly. In the M/S encoding scheme, a signal which is correlated between the channels being removed is generated by converting two-channel signals of the L signal and the R signal into a multiplication signal (hereinafter, referred to as an M signal) between the L signal and the R signal and a subtraction signal (hereinafter, referred to as an S signal) between the L signal and R signal. In the M/S encoding scheme, the signals are encoded after the correlation between the channels is removed from the signals. As a result, it is possible to perform encoding efficiently by reducing the redundant information contained in the two-channel signal prior to the conversion. In addition, there is a known technique called a parametric stereo encoding scheme which uses the correlation between the two-channel signals of the L and the R signals. In the parametric stereo encoding scheme, the two-channel signal including the L signal and the R signal is represented according to a one-channel signal and a parameter indicating a relationship between the channels. The one-channel signal and the parameter for expanding the channels signal are encoded.
In addition, various techniques have been developed up to now for a process of suppressing sound quality deterioration caused when an erroneous transmission occurs in a multi-channel encoding/decoding scheme.
Patent Literature 1 discloses a technique of suppressing the allophone caused by an abrupt change in the number of channels in the decoded signal when frame loss is generated by a transmission error and the like in the multi-channel signal parametric encoding scheme. Specifically, in Patent Literature 1, when the frame loss occurs, a process for generating a substitution signal for the wrong parts based on the stored parameter relating to the signal having no fault is performed. Patent Literature 1 discloses a process of applying stepwise muting of the model parameter when the defective frames are continued in series.